1. Field of the Invention
Polyimide is a polymeric plastic material. Due to its excellent engineered characteristics on thermal and chemical resistance as well as its stability for prolonged periods of time, it is widely applied in many demanding industrial applications such as electronics, aerospace, and electrical industries when a high temperature material is required. In many modern microelectronics and micromachining devices, the thin/thick films cured from applicable liquid polyimide have been broadly incorporated into various microstructures as an essential material.
The present invention discloses the details of a new method and process to form metal interconnections on micromachining devices which is integrated with thick polyimide films according to the preamble of the independent claims. It concerns to provide a reliable metallization method between circuits on polyimide and other layers by micromachining approach of manufacture. More particularly, the embodiments in the current invention will present micromachining techniques to form a multi-steps staircase structure with tapered angle on polyimide films, which can be applied to achieve good step coverage for metal interconnection among different level of circuits.
2. Description of the Related Art
Over the past few years, various prior arts related to slope etching of polyimide, and more particularly, to slope etching of polyimide for forming metallization method in an integrated circuit had been heretofore developed and disclosed.
The U.S. Pat. No. 4,832,788 (Method of Fabrication a Tapered Via Hole In Polyimide; by Michael H. Nemiroff, et al.) reveals a method of fabricating a tapered via hole in a polyimide layer by underlying a thin layer of silicon dioxide (SiO2) between photoresist and polyimide layer. The silicon dioxide (SiO2) layer combined with photoresist are used to create slope etching on polyimide by multi-stage plasma etching of these two film to enlarge the opening. However, its drawback is that the sidewall surface of the tapered step on polyimide would appear zigzag-like roughness which could cause discontinuity on metallization if the roughness is not controlled properly.
The U.S. Pat. No. 4,487,652 (Slope Etch of Polyimide; by Carl W. Almgren) teaches a method of fabricating a sloped via hole through polyimide by first sloping a hard mask which overlies the polyimide, and thereafter the sloped hard mask is used to slope the polyimide. The limitation of the revealed method is that the etching selectivity between oxide and polyimide is about 1 to 3. Therefore if the polyimide film is very thick like more than 1.0 um, then the corresponding thickness of oxide layer will need more than 3 um for the slope etching which is not feasible in reality. Moreover, in order to slope-etch the oxide layer, the overlaid photoresist needs reflow in which the process will cause the feature size change of via.
A wet etching method to generate etched slope on polyimide layer is revealed in the U.S. Pat. No. 4,369,090 (Process for Etching Sloped Vias in Polyimide Insulators; by Arthur M. Wilson et al.). Since the wet etching rate of polyimide films depends on its curing temperature and curing time, thus the sloped via could be achieved by etching through of several layers of partially cured polyimide. The slope angle of via could be indeed well controlled by this method; however, it is as well apparent that the complex curing process for different layer of polyimide is costly and infeasible in real applications.
Compared to aforementioned prior arts, the embodiments of the current invention will present simple and complete solutions to achieve reliable metallization on polyimide films, especially for those ultra thick of polyimide films incorporated in micromachining devices.